Nitric oxide (NO) is a diatomic free radical endogenously synthesized in the human body when L-arginine is converted to L-citrulline by a class of enzymes known nitric oxide synthases (NOS's). Since the first reports describing NO's action as an endothelium-derived relaxation factor, much research has been devoted to elucidating the pathways of NO generation and action in biological milieu. In particular, NO is involved in angiogenesis, wound healing, platelet activation, neurotransmission, vasodilation, immune responses, the inhibition of platelet aggregation, and in blood pressure control. See Zhang, X., Frontiers in Bioscience, 9, 3434-3446 (2004).
Sepsis is the 10th leading cause of death in the United States, and the leading cause of death in non-cardiac intensive care units (ICUs). Sepsis often originates from medical device infections, and severe cases account for 1 in 5 admissions to ICUs in the U.S. Furthermore, the onset of sepsis corresponds with increased levels of NO. Accordingly, motivation exists to improve the ability to detect the onset of sepsis by increasing speed, accuracy, and ease of diagnosis by NO detection devices and methods.
The detection of NO in blood may be used as a biomarker for sepsis, to evaluate wound healing by measuring NO in wound fluid, and to evaluate the efficacy and NO-release kinetics of pharmaceuticals that directly release, or modulate the endogenous release of NO. Challenges of in vivo biological NO detection may include biofouling (i.e., platelet/protein adhesion and clot formation), noise, and risk of infection. With regard to ex vivo detection of biological NO, challenges for such detection may include the transport of fluids, the reactivity of NO, fluid volume demands, and sensor drift.
Methods for measuring NO directly include chemiluminescence, electron paramagnetic resonance, spectroscopy, and electrochemistry. Measurement of NO with chemiluminescene and electron paramagnetic resonance may provide for more sensitive and direct measurements, but these methods are expensive and require extensive training for accurately measuring NO. Further, these methods for measuring NO are difficult when measuring NO in certain mediums, such as whole blood.
Miniaturized electrochemical sensors represent promising devices for determining the spatial and temporal distribution of NO in physiology, as they are readily miniaturized. Such electrochemical sensors, however, have a number of limitations and challenges that must be addressed, such as low sensitivity, comparatively slow response time, and/or interferences from other readily oxidizable biological species (e.g., nitrate, ascorbic acid, uric acid, dopamine, etc.). Furthermore, such electrochemical sensors require sample sizes greater than tens of milliliters for determining an amount of NO.
Accordingly, there is a need in the art for microfluidic sensors for measuring and detecting molecular species in smaller sample sizes, such as tens of microliters. Further, there is a need for microfluidic sensors that are highly selective for molecular species over other biologically relevant interfering species.